The invention relates to the manufacture of composite articles in an expedited manner and, more specifically, to an apparatus and method for inspecting composite articles during manufacture and automatically altering system parameters based thereon.
Composite structures made from fiber-reinforced polymer matrix (resin) materials are commonly manufactured by progressively building up the structure with a plurality of layers of thin composite tape or tow, hereafter collectively referred to as tape, laid one layer upon another. Typically, the operation begins by laying one or more tapes onto a starting template or tool that has a configuration generally corresponding to the desired shape of the article to be produced. A tape placement head guides the one or more continuous tapes onto the template by providing relative movement between the template and the head, such that the head moves over the surface of the template. The head usually makes repeated passes over the template in a defined pattern until the composite material is entirely collated. Multiple plies or layers of tape are built up by continued passes of the head over the surface. A compaction roller is usually used for pressing the tape against the template or prior-laid layers of tape, hereafter collectively referred to as a workpiece. Compaction facilitates adhesion of the tape to the workpiece. The tape, the workpiece, or both are heated just prior to the tape being compacted to soften the resin and promote adhesion of the tape to the workpiece.
Composite structures having a contoured shape can be formed by collating multiple layers on a contoured template. Alternatively, composite structures of varying geometry can be produced by first laying multiple layers of tape on a flat template in the manner described above. After a sufficient thickness is achieved, the composite material is heated and subsequently cured in the shape of a contoured template. Excess material is then removed.
A number of defects can occur during manufacture of composite structures by an automated collation process. For example, gaps between tapes, overlap of tapes, or twists in the tapes can result from unexpected variation in tape width or from improper or imprecise coordination of the placement of the fiber tapes. Such gaps, overlaps, and twists can have significant adverse effects on the strength and durability of the resulting composite material. In addition, adhesion problems can result from insufficient or excessive heating or compacting. Insufficient adhesion increases the likelihood of separation of the successive layers of tape, thereby decreasing the strength of the finished article. Concerns over maintaining adequate adhesion at higher rates are typically addressed by adding excess heat to the material. Unfortunately, it is oftentimes difficult to remedy adhesion problems since the ideal degrees of heating and compacting are mutually dependent and also dependent on such factors as the type of fiber tape employed, the degree of impregnation of the fiber tape, the rate at which the fiber tapes are placed, and ambient conditions, to name a few.
Some defects cannot be repaired, thus increasing the average effective cost per usable part produced. Other defects can be repaired but require costly and time-consuming human intervention. In traditional manufacturing methods, the composite structure is inspected after each layer of tape is placed. Since defects are generally detected manually, an element of subjectivity is disadvantageously introduced into the inspection process. Once defects are detected, a worker repairs the defects if possible. As described, this process requires human intervention during inspection and repair. The fiber placement machine may therefore be completely inoperable or at least slowed during the time required for inspection and repair, thereby reducing the output of the machine. As a result, a conventional fiber placement machine disadvantageously operates at a rate well below its maximum capabilities in most situations, limiting potential throughput.
The present invention seeks to improve the quality of composite materials and the efficiency of their production by providing an inspection system that provides feedback that is used by a controller to control various system parameters, including an advanced heat source. Integration of the inspection system enables a closed-loop control system that can quickly adjust system parameters to correct or avoid defects in the product, thereby enabling the composite material collation machine to operate much closer to its maximum capabilities than provided by conventional material collation machines. The system therefore offers significant reductions in manufacturing time and cost while also providing an anticipated improvement in quality.
Composite material collation machines include fiber tape placement machines that typically lay several tapes simultaneously as well as machines for placing other types of materials, such as individual tapes several inches wide. While embodiments of the present invention are described in the context of fiber placement, it is understood that each of the embodiments is applicable to all of the other machines and methods for composite material collation.
In one embodiment, the composite material collation machine comprises a fiber placement machine that includes a laser diode array that heats at least one fiber tape, and a compaction roller then compacts the fiber tapes onto a workpiece in a compaction region. The fiber tape therefore conforms to the contour of the workpiece and is adhered thereto. The fiber placement machine also includes an inspection system, typically comprising a camera and/or other sensors, that monitors the process and material and generates an output that is representative of at least one characteristic of the process, such as of the fiber tape or workpiece. By way of example, the inspection system may generate an output that is representative of the temperature of the fiber tape, the temperature of the workpiece, the rate of placement of the fiber tape, the compaction pressure, the tack of the fiber tape, and/or the placement of the fiber tape relative to another fiber tape. The fiber placement machine also includes a controller for processing the output of the inspection system. Based on the output from the inspection system, the controller may adjust at least one system parameter defining an operational characteristic of the fiber placement machine.
For example, the controller may adjust the power of the laser diode array, the temperature of the fiber tape, the temperature of the workpiece, the rate of placement of the fiber tape, the compaction pressure, the tack of the fiber tape, and/or the placement of the fiber tape relative to another tape. In this regard, if the controller detects a gap between two adjacent fiber tapes, the placement of those tapes could be adjusted so that they are placed closer together and/or the temperature and compaction pressure may be increased. If the controller detected lower than optimal adhesion of the tapes, the temperature could be increased by increasing the power of the laser diode array, increasing the compaction pressure, reducing the rate of lay of the tapes, or some combination thereof. Alternatively, the fiber tapes can be monitored before the fiber tapes are compacted with the controller similarly taking appropriate corrective action, albeit generally prior to the production of an unacceptable layer in this instance. For example, a temperature sensor can detect the temperature of the fiber tapes before the compaction device presses the tapes on the workpiece. This may occur either before, during, or after the fiber tape is heated by the laser diode array. The temperature sensor of this embodiment sends an output to the controller that is representative of the temperature at one or more points on the fiber tapes. The controller processes the output of the temperature sensor and adjusts the power of the laser diode array to maintain an appropriate temperature. Similarly, the temperature of the workpiece can be monitored and adjusted accordingly.
The fiber placement machine may also include a marking device capable of indicating defects on the fiber tape. For example, the marking device may spray or otherwise deposit an amount of ink, paint or the like onto the fiber tapes in those areas where defects have been detected. The marking on the fiber tapes enables the location of the defects to be subsequently readily identified either automatically or manually.
The inspection system may include a device for measuring the molecular mobility of the resin of the fiber tape. One way of determining the molecular mobility of the resin is to measure the nuclear magnetic resonance (NMR) T2 relaxation time. Since the molecular mobility, cure, and tack of the resin are related in a predefined manner, the tack of the tape can be monitored accurately and without intrusion based upon the molecular mobility of the resin. The inspection system may therefore include a tack-measuring device which can measure the tack before or after the tape is compacted. The tack-measuring device generates an output that is sent to the controller and the controller adjusts system parameters based on the output. For example, if the resin is detected to be above the preferred tack point or range of tack points, the controller can reduce the power to the laser diode array, decrease the compaction pressure, increase the rate of placement of the fiber tape, or perform some combination of these steps.
The automatic, closed-loop controlled system and method of the present invention can therefore operate faster and with fewer interruptions than conventional fiber placement systems because human intervention is not required for inspection and repair. Thus, the fiber placement machine of the present invention can operate closer to its maximum capacity than a traditional fiber placement machine. Also, by detecting defects quickly, the fiber placement machine and method of the present invention enables defects to be rapidly corrected by automatically adjusting one or more system parameters that define an operational characteristic of the fiber placement machine. In addition, some potential defects may be detected prior to their occurrence, thereby permitting the operational characteristics of the fiber placement machine to be adjusted to avoid the manufacture of a defective article. The result is a part built at a much higher rate with reduction in repair time of defective materials and an avoidance of defective materials that must be scrapped. Thus, the average cost per part is reduced due to the increase in operational efficiency of the machine and the reduction in repair time. Additionally, because defects can be detected more uniformly with the automated system and method than a traditional human inspection, the present invention enables an improvement in the overall quality of the parts produced.